1. Field of the Invention
This invention relates to a method and apparatus for rapidly evaluating the effectiveness of a corrosion fatigue inhibitor on a metal exposed to a corrosive environment while under stress.
Corrosion fatigue is one of the most common causes of oilfield production equipment failure. This failure mechanism is frequently manifested by failure of sucker rods in pumping wells using a sucker rod lift system. The sucker rods of pumping wells are often in contact with corrosive fluids such as brine and/or oil which contains hydrogen sulfide or carbon dioxide. Failure of a sucker rod in a pumping well results in an expensive workover operation and lost production.
The prevailing method of treating sucker rods against corrosion fatigue involves application of a corrosion inhibitor periodically or continuously. A large number of inhibitors are available commercially, and these inhibitors are effective to varying degrees depending upon many factors including the nature of the corrosive fluid in contact with the equipment, the type of material involved, and the conditions of use. While some of the available inhibitors are very effective in certain conditions, it is difficult to predict the effectiveness of a given inhibitor in a particular set of conditions.
2. Description of the Prior Art
The normal method for evaluating the effectiveness of a corrosion inhibitor involves exposing a test specimen of the material to be tested to a corrosive environment for a period of time, and then determining by weight loss the degree of corrosion. It is a relatively simple matter to compare the weight loss of a specimen exposed to a corrosive environment with the weight loss of a similar specimen exposed to the corrosive environment but protected by an inhibitor. However, the results of this type of testingdo not accurately correlate with the corrosion fatigue properties of the system under consideration. That is, a corrosion inhibitor may indicate excellent inhibition of corrosion by the weight loss method and yet the material in question, protected by an inhibitor, may fail due to corrosion fatigue at a time similar to or not much different from the time it would have failed if the inhibitor had not been used. This is apparently due to the fact that the type of corrosion which leads to corrosion fatigue failure is localized, and even though the inhibitor protects most of the surface of the material, there is sufficient breakdown of the protective film to allow localized corrosion fatigue cracks to develop, leading to early failure in spite of the presence of an inhibitor.
This discrepancy between the degree of inhibition indicated by the weight loss testing method and the actual corrosion fatigue inhibition has been recognized some time, and several attempts have been made to obtain corrosion fatigue inhibitor effectiveness data by exposing a specimen of material to be tested to a corrosive environment while dynamically stressing the specimen. One approach to determining the behavior of solid materials exposed to a corrosive environment while the solid materials are being dynamically stressed is described in U.S. Pat. No. 3,427,873. The apparatus and method described in that patent involve repeatedly flexing the material being tested while it is exposed to a corrosive environment. The procedure described in that patent works quite well, but is subject to the deficiency that the test specimen must be tested to failure, which in many cases involves several million cycles, before the performance of the inhibitor in question can be determined.
U.S. Pat. No. 3,504,535 describes an apparatus for simulating and monitoring the stress corrosion condition of a stressed member stored therein by measuring pressure in a tank containing the member.
Corrosion monitoring apparatus including a hydrogen permeable membrane and a getter-ion pump, which apparatus provides a current readout indicative of corrosion rate, is described in detail in U.S. Pat. No. 3,942,546.
There has been a need for a capability of determining the effectiveness of a corrosion fatigue inhibitor by a method which is reliable and can be performed in a short period of time. This invention provides that capability.